It has already been proposed to reduce the friction of air on the resonator in order to increase the quality factor of a regulator device of a clock movement.
In order to reduce the air friction on a resonator in the spring balance of a mechanical clock movement in particular, it is known from document FR 2054540 to incorporate this movement entirely in an air-sealed case, inside which the pressure is reduced to below atmospheric pressure to obtain a low pressure or reduced pressure in this case. To ensure the adjustment of the oscillation frequency of the spring balance, this document provides a system of two bimetallic blades acting on the classic index when one or other of these blades is heated either by an electric current supplied through electrical contacts arranged in the base of the case or by a light beam through a glass of the case.
In a second embodiment with an electronic movement it is provided to place an electromagnetic oscillator in a hermetically sealed chamber, in which this oscillator and the means for maintaining and regulating it are housed. The oscillation of the balance is maintained by two coils linked to magnets borne by this balance. The adjustment of the active length of the spring is conducted in a similar manner to the previous embodiment by an electric current supplied by bimetallic blades. As can be seen in FIG. 4 of document FR 2054540, the oscillator is connected to the rest of the electronic movement solely by electric connections through a wall of the chamber. It will be observed that the arrangement of electric connections across a hermetic chamber does not pose any particular problem.
It is therefore noted that document FR 2054540 published in 1971 instructs placing the oscillator in its own hermetic chamber in the case of an electronic movement, whereas in the case of a mechanical movement it is provided to incorporate the entire clock movement in a sealed case and to reduce the pressure in this watch case. It can be noted that this instruction has prevailed in the clockmaking field. Since the proposed embodiment for an electronic movement is relatively simple to manufacture, in particular for a quartz resonator, this instruction has become established for electronic movements. In contrast, in the case of mechanical movements various significant problems have not enabled mechanical watches to be fabricated commercially with their movements each housed in a hermetic case, in which a reduced pressure prevails.
Firstly, mechanical watches generally have mechanical elements that pass through the case for the adjustment and/or operation of various functions, and this complicates the design of the case to enable this to ensure that a relatively stable reduced pressure is maintained over a long period. Then, in the event of an after-sales service that requires that the case is opened, there must be the means available to once again provide an air void in this case until a low pressure is produced. Moreover, since the adjustment of the frequency of the resonator depends on the ambient pressure, this adjustment poses a manufacturing problem. The solution comprising acting on an index using bimetallic blades by supplying an electric current or a light beam via the base of the case once the movement is encased and the low pressure has been established in particular poses a problem with respect to the precision of such an adjustment and a problem in the production process, since the individual adjustment of each watch must be done once the movement has been encased.
Recently document WO 2013/084040 has sought to solve the problem of the adjustment of the balance spring of a mechanical watch having a watch case, in which a reduced pressure is provided to improve the quality factor of this oscillator. This document proposes a solution, in which the adjustment is conducted at ambient pressure while taking into account an adjustment difference measured for operation at ambient pressure and operation at a determined reduced pressure. However, it will be observed that this document remains within the concept of the previous document by proposing to place the clock movement entirely in a watch case, inside which an air void is provided to obtain the desired reduced pressure. Hence, apart from a process that enables a particular device in the watch to be omitted in order to conduct the adjustment of the working thereof under reduced pressure, all the other problems remain.